With reference to FIG. 1, a conventional loudspeaker 20 generally comprises a support frame 22, a cone 24, a dust cap 26 bridging across the cone, a suspension system, a voice coil 40, a voice coil former 43, and a vented pole piece 41. The voice coil 40 is wound about the voice coil former 43 such that an annular magnetic gap is defined between a top plate 37 and the magnet and the voice coil 40.
The magnetic circuit linearly cycles or displaces the voice coil former 43 in this gap. In the conventional speaker the cone 24 is attached to the voice coil former above the coil 40 at its lower end and to the frame at its upper end. A suspension system comprising two elements connects to the frame and upper end of the cone, and to the frame and voice coil former, respectively.
The suspension system of the loudspeaker normally comprises two elements, the surround 28 (upper or outer suspension) and the spider 38 (lower or inner suspension). The surround 28 is a mechanical device which holds the outer edge of the diaphragm/cone of the loudspeaker. The surround 28 functions to center the cone 24, and to provide a portion of the restoring force that keeps voice coil in the gap defined between the pole piece and the top plate of the loudspeaker. The spider 38 also helps to keep the voice coil concentric to the pole piece, and it provides another component of the restoring force that maintains the voice coil within the gap.
Often a loudspeaker design can be best optimized by utilizing a voice coil with a smaller diameter. However, the smaller voice coil setup creates certain problems, especially when designing loudspeakers for low frequency reproduction. Thus, for larger diameter loudspeakers (typically 10 inches and above), small voice coil systems are not common. Accordingly, there are few, if any, existing cones tooled for the smaller diameter coil former. To incorporate a small voice coil system, the cone must be customized, adapted or re-tooled.
One disadvantage of mating a cone directly to a smaller voice coil is that a relatively small adhesive joint is made. Since the voice coil's diameter is much smaller, the gluing circumference is drastically reduced. Therefore, the designer must be concerned with the possibility of mechanical failures since the stress distribution around the glue joint is high. Because the spider attaches at this critical junction as well, spider joint stress also increases, introducing yet another possible failure mode.
Another problem associated with smaller voice coils occurs in the use of pole vents. Pole vents comprise holes bored directly through the pole piece within the motor structure. These vents are used to relieve air pressure that builds up beneath the dust cap. Without a pole vent, audible noise can be introduced as the trapped air tries to escape during large cone excursions. However, when using a small diameter voice coil, the amount of metal in the pole piece is very limited. This amount of steel can only support limited amount of magnetic flux. Consequently, using a pole piece with large amounts of metal removed for pole vents can radically alter the performance of the magnetic circuit.
A vented pole piece further effects the thermal behavior of the speaker. The steel contained in the pole piece provides an effective thermal sink for the voice coil. Machining a pole vent in the pole piece increases thermal resistance of the sink, lowering the power handling capability of the loudspeaker.
The mechanical integrity of the spider is also comprised when using small voice coil. Spiders are typically made from resin treated cloth materials. When the inner diameter of the spider gets smaller, fewer strands of material intersect the cutout. Since the glue joint lies on this small circumference, very little spider material is captured. This places the spider material under greater stress than normal. This high-stress condition could cause the spider itself to fatigue prematurely. Since the spider is typically called on to center the moving assembly and limit cone motion at the extremes of excursion, a comprised spider could cause a catastrophic failure.
Resistance to rocking is reduced when using a smaller inner diameter voice coil. The term "rocking" refers to tilting of the voice coil toward the pole piece and/or top plate within the gap between the two. As a spider's inner diameter gets larger, the material along the inner diameter is required to deflect more when the moving assembly rotates a given amount (as during rocking). Consequently, a spider with a larger inner diameter will be more resistant to rocking because more energy is required to invoke a given angular change. It follows that using a small voice coil, and hence a small inner diameter spider, makes a given loudspeaker more susceptible to rocking related problems.
The smaller voice coil system further affects the cone's structural integrity. As a voice coil gets smaller, the cone angle increases (using a vertical axis as a reference), causing the cone to become flatter. As the cone begins to flatten, its mechanical strength drops. Increasing the cone angle increases the likelihood of audible degradation due to cone flexure. Normally, the only option available for preventing cone flexure is to increase the cone thickness and/or increase the cone depth. This decreases the cone angle and makes the cone wall more vertical. These solutions, however, are not desirable since increasing the cone depth requires a larger frame depth and using a thicker cone adds weight to the moving structure. Moreover, thicker cones and deeper frames require special tooling and make the speaker's mounting depth unattractive for certain applications.
Several loudspeaker designs are contemplated in the background art for improving speaker performance, stabilizing the speaker cone/diaphragm, and/or simplifying the manufacturing process. However, none of these references solve the above-noted problems. For example, Mitobe (U.S. Pat. No. 5,111,510) discloses a speaker and manufacturing method therefor including a diaphragm integrally combined with a first frame piece and a driver unit integrally combined with a second frame piece. Saiki et al. (U.S. Pat. No. 5,371,805) discloses a speaker and speaker system employing the same, comprising a diaphragm secured to a first periphery of an edge member and a frame secured to a second periphery of the edge member. Scholz (U.S. Pat. No. 5,323,469) discloses a conical loudspeaker having a conical stabilizing element joined between an underside of a speaker membrane and an outside surface of a speaker moving coil carrier. Kreitmeier (U.S. Pat. No. 5,424,496) discloses an electromagnetic converter comprising an internal magnet system, a moving coil and tubular segment. Kreitmeier (U.S. Pat. No. 4,764,968) discloses a disk-like diaphragm made from a conical plastic film and provided with vacuum formed support members which extend up to the disk-like radiating layer. Finally, Kobayashi (U.S. Pat. No. 4,118,605) discloses a coil mount structure comprising a cylindrical member, around one end portion of which a diaphragm edge is fixed, an inner peripheral edge portion where a damper is removably fixed, and an opposite end portion around which a coil is provided. Kobayashi, however, does not provide any structure for ventilating air pressure from beneath the dust cap or a structure for creating a secure joint between the diaphragm/cone, spider, and/or voice coil. The present invention, by way of contrast, is directed to an adaptor ring, the structure of which facilitates a stronger adhesive joint between the cone, spider, and voice coil bobbin or former, and means for venting air pressure buildup.
The above-noted background art neither solves or addresses the problems contemplated by the present invention. Accordingly, there remains a need for a loudspeaker capable of providing improved structural joints between the speaker cone, spider, and voice coil former, allowing the use of smaller voice coil systems and providing ventilation in the speaker without forfeiting performance.